1. Field of the Invention
This invention relates to a traction control system for a vehicle, and, more particularly, to a traction control system suitable for a vehicle equipped with an exhaust system including a catalytic converter.
2. Description of Related Art
In order for automotive vehicles to prevent deterioration in acceleration performance due to slippage of driving wheels which is caused by excessive driving torque, it is typical to perform traction control where engine output or braking force applied to driving wheels is controlled so that slippage of the driving wheels, which is calculated based on rotational speeds of the driving wheels, reaches a desired or target amount of slippage. In a traction control system of this kind, in order to decrease engine output, the ignition timing may be retarded and/or the fuel supply to specific cylinders may be restricted according to slippage of the driving wheels. Specifically, the engine output is controlled according to control levels which specify engine control patterns each of which defines retardation of ignition timing and/or the number of cylinders for which fuel supply is to be restricted according to slippage of the driving wheels.
In the traction control system of this kind, the retardation of ignition timing causes the engine to increase the amount of unburned ingredients in the exhaust gas and, in addition, provides a tendency to continue what is called an "after-burn" for a relatively long time which leads to an increase in the temperature of the exhaust gas. Additive effects these changes cause make oxidization of the unburned ingredients in the exhaust with the catalyst very active and, as a result, potentially produce a rise in the temperature of the catalytic converter. This deterioration in the exhaust gas purifying performance of the catalytic converter and aggravation of durability of the catalytic converter. Together, the suspension of fuel delivery to only specific cylinders allows unburned ingredients discharged from the remaining cylinders to which fuel has been delivered to be potentially brought into contact with concentrated oxygen in the exhaust from the specific cylinders before reaching the catalytic converter. This contact of the unburned ingredients with oxygen leads to reheating the exhaust and causes the same problems.
Various efforts have been made to eliminate aggravation of performance of the catalytic converter. One such effort is that described in Japanese Laid-Open Patent No. 5-1613. The approach used is to alter an engine control mode from an ordinary control mode to a specific mode, such as an exhaust gas temperature reduction mode, in which the amount of unburned ingredients in the exhaust entering into the catalytic converter is reduced as low as possible when the temperature of a catalytic converter becomes higher than a predetermined temperature. This teaching alludes to generalized techniques for assuring the interruption of retardation of ignition timing when the catalytic converter is at high temperatures, so that the catalytic converter is prevented from suffering an extraordinary rise in temperature, thereby reducing or avoiding aggravation of durability.
While the system as described in Japanese Laid-Open Patent No. 5-1613 may have advantages over the prior art, nevertheless, a certain constraint must be imposed upon the control of engine output. That is, in the exhaust gas temperature reduction mode, in order for the engine to reduce the engine output one step down, the engine output control is performed by means of interrupting fuel delivery instead of retarding ignition timing, resulting in rough changes in engine output. This causes a sharp drop in driving force delivered to driving wheels, giving the driver a feeling of stall.
Further, a temperature sensor for detecting the temperature of the catalytic converter is essential within the system which is not always desirable in terms of costs.